A field data set collected under different conditions is analyzed to characterize the spatial arrangement of two large inflows (Ebro and Segre) with distinct physical-chemical characteristics as they join at the upstream end of Ribarroja reservoir in northern Spain. Given the short average residence time of water in the reservoir, the spatial arrangement of the rivers at their confluence and their mixing rates are likely the drivers of the stratification patterns observed near the dam. In winter, inflows have similar densities—Δρ/ρ0 ≈ O(10−5)—and their spatial distribution is largely determined by inertial forces, and in particular, by the discharge ratio. Downstream of the confluence, both rivers flow side by side and largely unmixed over long distances. In summer, with Δρ/ρ0 of O(10−3), the flow fields at the confluence are largely controlled by buoyancy forces. Atmospheric forcing during strong wind events and centrifugal forces caused by the meandering shape of the reservoir induce significant tilting of the isotherms, leading to localized high mixing rates. Mixing, in general, though is weak at this time of the year. In fall and early winter, density differences are largely controlled by conductivity differences between the incoming flows. The warmer Ebro water, with larger thermal inertia, flows beneath the colder Segre water. The spatial arrangement of the inflows is largely controlled by the discharge ratio and mixing between sources is strong, likely as a result of mixed water being denser than either of the incoming flows.